All electrical cables are covered in some form of insulation. The copper or aluminium wires conduct electrical current to power your equipment, and insulation works to do the opposite of the conducting material, ie. it resists current and protects the user by keeping the current in its intended path along the wire or cable. Insulation testing is a common and very important form of electrical testing so that we can ensure cables are safe to use.
How does insulation work?
The key to insulation testing is understanding what resistance is, and we can do this by considering water flow. If water is pumped along a pipe under pressure, it will flow okay, but the flow will slow slightly because of friction from the internal walls of the pipe, and if there is a leak, then pressure will drop considerably and we will lose all the water. With electrical current flow, the voltage replaces the water pressure and this enables current to flow, but we get resistance in the current flow from the insulation, and if there is leaking current the voltage will drop and we will lose charge to our equipment, we will also face considerable safety issues because there is live current escaping through the insulation.
No insulation is perfect, however, so some current will flow along the insulation, and as long as it does not exceed a certain value, the insulation is deemed to be adequate and the cable is safe to use. If insulation is producing a low resistance, however, this is a problem, ie. too much current is escaping because the cable could be old and damaged, or there is an excessive voltage being passed through it.
Testing insulation resistance
We use Ohms Law to test insulation resistance, which is E = I x R, where E is the incoming voltage in volts, I is the current in amps and R is the resistance in ohms.
Using a multi-function tester, such as the MFT-Pro supplied by Test Instrument Solutions, we can test the insulation resistance of a cable on a routine basis.
- Make sure the multi-function tester is in calibration
- Carry out a visual check of the unit even if it is within the calibration date
- Check the condition of all the tips and probes
- Turn on the machine and set it to the ohms scale
- Zero out the green and red leads by inputting them into the correct ports and touching the tips together to measure any resistance in the leads themselves, ie. which would skew the results. Press the TEST button to zero these out.
- Set the dial to 500 volts DC in the megaohms scale to test a 230 volts or 400 volts circuit
- With the green and red cables inputted short the leads together to prove you have a dead short. This is where you match the crocodile clips together along the maximum surface area, and you should see a zero reading on the display when you press the TEST button, to show you have no breaks in the cables.
- Testing for insulation resistance
- Disconnect any live conductors from the distribution board
- Connect the test lead to the line conductor and the other lead to the neutral conductor
- Set the tester to the required voltage and press/hold the TEST button
- The resistance value is displayed in ohms
- Repeat the test with the line and earth conductors connected
- Repeat the test with the neutral and earth conductors connected
- Ideally you should see results no lower than 1 megaohm. This shows you have adequate resistance in the cable. If you have results lower than that, you need to isolate usage of the cables and carry out suitable investigations.
Please note that this section is for information purposes only. Anyone using equipment referred to in this section must be suitably qualified and/or experienced within the respective field. If in doubt before use, please consult a qualified electrician or engineer & thoroughly read all instruction booklets.